Summary of Work: The p53 protein is critical for cell function and maintaining the integrity of the genome in response to environmental stresses, including chemical and physical carcinogens. Reduced p53 gene dosage (heterozygous p53 deficient mice)by itself is insufficient to induce genomic instability and to cause cancer. However, we, and others, have demonstrated that p53 deficient mice exposed to chemical and physical carcinogens rapidly develop cancer. Gaining insight and understanding the mechanistic basis for induction of organ/tissue specific cancer using model carcinogens is important to developing methods for carcinogen identification and minimizing exposure as well as developing public health strategies for intervention and prevention. The carcinogens, p-cresidine, benzene, and phenolphthalein each rapidly induce site specific tumors (bladder, histiocytic sarcoma, or thymic lymphoma, respectively) in heterozygous p53 (+/-) deficient C57BL/6 mice (het p53 def). The findings are consistent with the hypothesis that inactivating mutations or LOH involving the trp53 locus are critical in rodent as well as human cancers. Each carcinogen induced tumors that demonstrated different levels of wild type p53 allele loss (phenolphthalein > benzene > p-cresidine) in the het p53 def mice. With either high frequency (benzene) or complete (phenolphthalein) loss of the wild type allele (Southern analysis) no inactivating mutations in p53 could be found. However, in the case of the aromatic amine induced bladder tumors, we failed to observe inactivating mutations in p53 exons 5-9 by either p53 immunohistochemistry, cold SSCP analysis, or limited sequencing of PCR amplified exon 5-8. Although, using mice of the same strain carrying the lacI neutral reporter gene we were able to demonstrate that the bladder was mutagenized. These data demonstrates the dramatic difference in carcinogen and tissue specific p53 allele loss. Thus, we have established models to investigate the relationship between p53 gene dosage, inactivating mutations, and the potential mechanism of chromosomal instability.